1. Field of the Invention
The present invention relates to an optical amplifier of which amplifying medium is a rare earth-doped optical fiber. The present invention particularly relates to an optical fiber amplifier in which optical amplification is performed by maintaining polarization of a signal light in order that a gain can be improved and noise can be reduced. The present invention also particularly relates to an optical amplifier which can output an amplified light having superior linearly polarized light property.
2. Description of Related Art
For an optical amplifier using an optical fiber, an optical fiber amplifier of which amplifying medium is an erbium-doped optical fiber (hereinafter called EDF for short) is widely used.
When this optical amplifier is used combined with an optical unit having large polarization dependency such as optical modulator, polarization state of the light which is input to or output from such optical modulator is an important factor. In this kind of optical amplifier, linearly polarized light is used for an input light or an output light, a polarization maintaining optical amplifier is used for the purpose of maintaining polarization of the signal light. Such a polarization maintaining optical amplifier is reported in Japanese Unexamined Patent Application, First Publication No. H7-142798, Japanese Unexamined Patent Application, First Publication No. H11-112065.
In FIG. 6, a structure of a conventional optical fiber amplifier is shown. In FIG. 6, a reference numeral 1 indicates a signal light source. The signal light source 1 is connected to a light transmission path 2. This light transmission path 2 is connected to an input port of a first optical isolator 3a. At an output port of the first optical isolator 3a, an end of EDF 4 as an amplifying medium is connected.
Another end of the EDF 4 is connected to an input port of a wavelength-division-multiplexing coupler 5 (hereinafter called WDM coupler 5). To another input port of the WDM coupler 5, a pumping light source 6 is connected. To an output port of the WDM coupler 5, an input port of a second optical isolator 3b. The output port of the second optical isolator 3b is connected to an optical transmission path 2.
In an optical fiber amplifier, a pumping light from the pumping light source 6 and the signal light from the signal light source 1 are mixed by the WDM coupler 5 and input to the EDF 4, and optical amplification is performed therein. Such an amplified signal light is output to the light transmission path 2.
If a polarization maintaining optical unit and a polarization maintaining optical fiber are used for an optical unit organizing such optical fiber amplifier and an optical fiber which connects these optical units, light amplification can be performed while maintaining polarization.
However, in this optical fiber amplifier, because amplified spontaneous emission which is emitted from erbium which is doped to the EDF 4 transmits in the optical fiber; therefore, the amplified spontaneous emission is also amplified.
If population inversion at a rear end where a signal light is incident at EDF 4 becomes deteriorated due to the amplified spontaneous emission, a gain of the signal light becomes saturated; thus, it is difficult to obtain high gain. Also, because pumping light energy is used for amplifying the amplified spontaneous emission, the pumping efficiency worsens. Furthermore, population inversion at an end where signal light is incident is deteriorated, noise figure deteriorates.
Also, there is a problem that polarizing degree of the signal light deteriorates due to polarization cross talk which is caused when the number of the optical units which organize the optical amplifier and the number of the fusion splicing connection of these optical units increase.
In this kind of optical amplifier, it is not a problem if polarization state of the input signal light is an ideal linearly polarized light. However, if polarization component in which an orthogonal direction against this polarization direction is included, this unnecessary orthogonal light component is amplified; thus, unnecessary polarization light component is included in an output signal light. This means that the ratio of leaking light from the linear polarized light to an orthogonal component thereof when the linearly polarized light is incident (Such a ratio is called heareinafter a xe2x80x9ccross talkxe2x80x9d for short) increases and the performance of the polarization maintaining optical amplifier deteriorates.
The present invention was made in consideration of the abovementioned problems, and an object of the present invention is to provide an optical fiber amplifier which can improve gain and reduce noise figure by maintaining polarization of the signal light and amplifying it.
Another object of the present invention is to provide an optical amplifier in which light having unnecessary polarization component should not be included in the signal light.
In order to achieve abovementioned objects, a first aspect of the present invention is a polarization maintaining optical fiber amplifier which comprises an amplifying medium which is a rare earth-doped polarization maintaining optical fiber, a pumping light source which excites the rare earth-doped optical fiber, a linearly polarized light transmitting unit, wherein, the rare earth-doped optical fibers are connected in multi-stage such as two stages or more, the linearly polarized light transmitting unit which transmits linearly polarized light of either one of a slow axial directional component which is a polarization component of which transmitting speed is slow when being transmitted the polarization maintaining optical fiber, or a fast axial directional component which crosses with the slow axial directional component orthogonally and which is a polarization component of which transmitting speed is fast is disposed in at least one connected region in the rare earth-doped optical fiber.
A second aspect of the present invention is a polarization maintaining optical fiber amplifier of which linearly polarized light transmitting unit is a polarizer.
A third aspect of the present invention is a polarization maintaining optical fiber amplifier wherein a linearly polarized light transmitting unit is a polarization dependent polarization maintaining optical isolator which passes a linearly polarized light of either one in slow axial direction or in fast axial direction and does not pass a linearly polarized light of which polarization component is orthogonal to the above transmitted linearly polarized light.
A fourth aspect of the present invention is a polarization maintaining optical fiber amplifier wherein a rare earth-doped optical fiber is an erbium-doped optical fiber.
A fifth aspect of the present invention is a polarization maintaining optical fiber amplifier wherein optical units such as a rare earth-doped optical fiber, a pumping light source, and a linearly polarized light transmitting unit are connected by a polarization maintaining optical fiber.
A sixth aspect of the present invention is an optical amplifier which comprises an amplifying medium which is made of polarization maintaining optical fiber to which core a rare earth is doped, a pumping light source which excites the polarization maintaining optical fiber, a first linearly polarized light transmitting unit which is connected to an input of the polarization maintaining optical fiber, a second linearly polarized light transmitting unit which is connected to an output of the polarization maintaining optical fiber; wherein the first linearly polarized light transmitting unit transmits linearly polarized light of either one of a slow axial directional component which is a polarization component of which transmitting speed is slow when being transmitted the polarization maintaining optical fiber, or a fast axial directional component which crosses with the slow axial directional component orthogonally and is a polarization component of which transmitting speed is fast, a second linearly polarized light transmitting unit has almost same optical properties as the first linearly polarized light transmitting unit and transmits only a linearly polarized light which is transmitted through the first linearly polarized light transmitting unit. By employing such a structure, it is possible to perform an optical amplification while maintaining a linearly polarized light state of the signal light and prevent a light such as an unnecessary polarization component from being amplified.
A seventh aspect of the present invention is an optical amplifier wherein erbium is doped to the polarization maintaining optical fiber.
An eighth aspect of the present invention is an optical amplifier wherein optical units such as an amplifying medium, a pumping light source, and a linearly polarized light transmitting unit are connected by a polarization maintaining optical fiber.
A ninth aspect of the present invention is an optical amplifier wherein at least one of a first linearly polarized light transmitting unit, or a second linearly polarized light transmitting unit is a polarizer.
A tenth aspect of the present invention is an optical amplifier wherein at least one of a first linearly polarized light transmitting unit, or a second linearly polarized light transmitting unit is a polarization dependent optical isolator which transmits only either one of the linearly polarized light and does not transmit the linearly polarized light made of polarization component which is orthogonal to the above transmitted linearly polarized light.
An eleventh aspect of the present invention is an optical amplifier wherein a pumping method using a pumping light source employs any one of following pumping methods such as forward directional pumping method in which an pumped light is incident and pumped in a same direction as transmitting direction of the signal light, or backward directional pumping method in which an excited light is incident and pumped in an opposite direction to the transmitting direction of the signal light, or unilateral pumping method in which an excited light is incident and excited in both the same direction and the opposite direction of the signal light.
A twelfth aspect of the present invention is an optical amplifier wherein at least one polarization maintaining optical splitting coupler which distributes a part of an input light or an output light as a monitored light at an input of a first linearly polarized light transmitting unit or at an output of a second linearly polarized light transmitting unit. According to such a structure, it is possible to perform optical amplification while monitoring polarization state of the input light and the output light and maintain the optical power stable and output the amplified light of which linearly polarized light is maintained.
A thirteenth aspect of the present invention is an optical amplifier wherein a length of an erbium-doped polarization maintaining optical fiber is 320 m or shorter.
A fourteenth aspect of the present invention is an optical amplifier wherein an erbium-doped polarization maintaining optical fiber has two stress applying parts which are disposed in a clad which is disposed outside of a core so as to be symmetric against the core.
As explained above, according to the first embodiment of the present invention, by disposing a linearly polarized light transmitting unit which is a polarizer or a polarization dependent polarization maintaining optical isolator between two EDFs, it is possible to restrain the intensity of the amplified spontaneous emission and prevent population inversion of each EDF at a signal light incidence end from deteriorating; therefore, it is possible to realize a polarization maintaining optical fiber amplifier which can prevent the noise figure from deteriorating.
Also, because it is possible to reduce the power of the amplified spontaneous emission which is incident to the EDF by 50%, it is possible to reduce power of the excited light which is consumed for amplifying the amplified spontaneous emission; thus, it is possible to realize a polarization maintaining optical fiber amplifier which can amplify the signal light efficiently.
Furthermore, it is possible to realize a polarization maintaining optical fiber amplifier which can improve polarization cross talk which is generated while the signal light propagates optical units and connected points of each optical unit organizing an optical amplifier.
Furthermore, according to the second embodiment of the present invention, because light amplification is performed with maintaining a linearly polarized light state of the signal light by using a polarizer or a polarization dependent optical isolator; therefore, it is possible to realize an optical amplifier which can prevent a light which comprises polarization component from being amplified and to output the amplified light of which linearly polarized light state is maintained.
Also, by performing light amplification while monitoring the power of the input light and the output light via a polarization maintaining optical splitting coupler, it is possible to realize an optical amplifier which can output the amplified light of which linearly polarized light state is maintained while maintaining the optical power stable.